Catalysts comprising a titanium catalyst component and an organoaluminum compound have been commonly known hitherto as those for preparing polyolefins. Particularly, catalysts using a carrier-supported solid titanium catalyst component as the titanium catalyst component are known to exhibit a high polymerization activity.
Above all, the catalysts using a titanium catalyst component supported on magnesium chloride as the solid titanium catalyst component are known to exhibit a high polymerization activity and to produce polyolefins of high Istereoregularity when .alpha.-olefins of 3 or more carbon atoms are polymerized.
In this connection, various catalysts capable of producing polyolefins of higher stereoregularity (sometimes referred to as "highly stereospecific catalysts" hereinafter) have been proposed. For example, there has been proposed a catalyst formed from a solid titanium catalyst component supported on magnesium chloride which contains titanium, magnesium, halogen and an electron donor, an organoaluminum compound and an electron donating compound (electron donor) as a third component. Specifically, there has been proposed a highly stereospecific catalyst formed from a solid titanium catalyst component in which a titanium compound and a carboxylic acid ester as an electron donor are supported on magnesium chloride, an organoaluminum compound and a silicon compound having Si--OR (R is a hydrocarbon group) as the electron donor of the third component.
Meanwhile, Japanese Patent Laid-Open Publication No. 83006/1983 discloses that, in the preparation of the solid titanium catalyst component by contacting a magnesium compound, a titanium compound and an electron donor, if the magnesium compound and the titanium compound are contacted in a liquid state, the resulting solid titanium catalyst component shows high activity and high stereospecificity.
The present inventors have further studied such solid titanium catalysts showing high activity and high stereospecificity as mentioned above. As a result, they have found that a solid titanium catalyst component having an extremely high polymerization activity and showing high stereospecificity can be obtained by contacting a liquid magnesium compound, a liquid titanium compound, an electron donor and a solid divalent metallic halide. Based on the finding, the present invention has been accomplished.